Open-field production of 524,000 irrigated acres of horticultural plants in the United States used 205 billion gallons of water in 2013 and ≈50% of this water was pumped from groundwater sources (U.S. Department of Agriculture, 2014). Four states with large horticultural industries California, Florida, Oregon, and Texas used 60% of the 205 billion gallons of water. Water resources for irrigation are becoming increasingly limited so that technologies to conserve water are needed (Majsztrik et al., 2017).
Although sprinkler irrigation is used to produce plants in small containers (<28 cm diameter) in high densities, direct application of water using spray-stake irrigation is used to produce plants in larger containers that are placed in low densities. Compared with in-ground production, container production of plants with sprinkler irrigation is inherently inefficient, as containers occupy only a fraction of the production area even when closely spaced. Direct application of water to the container with spray-stake irrigation also can be inefficient, as typical water delivery rates for spray-stakes (15–40 cm/h) are much higher than for typical sprinkler systems (0.8–1.5 cm/h) so that small changes in irrigation run times can equate to large changes in application volumes and higher chances of overwatering (Million and Yeager, 2018b), particularly in bark-based substrates (Hoskins et al., 2014). Also, retention of water by the container substrate may be reduced at high application rates (Warren and Bilderback, 2005). Efficiency of spray-stake irrigation can be improved by using a cyclic irrigation schedule that applies water multiple times per day vs. a single application (Beeson and Haydu, 1995; Ruter, 1998).
Producing plants in large containers with spray-stake irrigation requires keen attention to detail if irrigation water is to be applied efficiently. To apply water efficiently, the irrigation system must be reliable, deliver water uniformly within the irrigation zone, and application rates should not vary greatly from one day to another. Even if the irrigation system delivers water consistently and uniformly, if irrigation needs within the irrigation zone vary due to nonuniform plant production conditions, such as varying plant species, stages of production, container sizes, container spacing patterns, and container substrates, efficient irrigation will be even more difficult to attain (Warren and Bilderback, 2005). Weather is another variable, as solar radiation, air temperature, and wind affect evapotranspiration (ET) rates and rain can reduce the irrigation demand (McCready et al., 2009). Beeson (2010) described the relationship between reference ET and actual ET based on plant canopy cover of the container production area.
The goal of efficient irrigation is to supply enough water for profitable production but not so much that unnecessary leaching occurs. One method for monitoring irrigation efficiency under a wide range of production conditions is to monitor the LF, the amount of leachate (container drainage) divided by the amount of irrigation water applied to the container. The LF can be routinely monitored and irrigation adjusted to achieve a desired LF. Stanley (2012) reported that implementing an LF monitoring program at a container nursery in Virginia reduced irrigation water use by >50%. For the largely sprinkler-irrigated nursery, a target LF of 10% was found to give good results throughout the nursery. Owen et al. (2009) reported that irrigation adjusted for a 20% LF with both pine bark-clay and pine bark-sand substrates did not return the substrate water content to container capacity. They reported a decrease in container weights of 0.6% to 0.8% per day if no rain occurred, indicating that over time and depending on the plant water needs, a water stress condition would occur unless additional irrigation water was applied, or rain received. Prehn et al. (2010) found that plants irrigated with a target LF of 20% produced similar-sized plants as those irrigated on-demand to maintain substrate moisture levels near container capacity. At a microirrigated container nursery, a target LF of 25% was found to be effective for a microirrigated container crop when irrigation was adjusted periodically (Million and Yeager, 2018a) but not when irrigation was adjusted daily based on ET and rain in the interval between LF tests (Million and Yeager, 2018b). The authors proposed that maintaining a fixed irrigation rate during the interval between LF tests may have allowed substrate moisture to “catch up” on days when the ET rate was less than the ET rate associated with the LF test day.
The objective of this study was to determine if the efficiency of an LF-based irrigation schedule could be improved with daily ET adjustments and if the improvement depended on the target LF value selected. Here, we describe two experiments, one with V. odoratissimum (L.) Ker Gawl. (Expt. 1) and one with P. macrophyllus (Thunb.) Sweet (Expt. 2), that monitored plant growth and water use as affected by irrigation schedule and the target LF.
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